1. Field of the Invention
The present invention relates to a fixing device for use in an image forming apparatus, such as a copying machine and a printer, and, more particularly, to a fixing device for use in an image forming apparatus capable of forming full-color images on both surfaces of a recording medium.
2. Related Background Art
A heat roller system has been widely used as a fixing device, in which fixing is performed for a recording medium that carries an unfixed image by clamping and conveying the medium between a pair of rotary members, at least one of which is heated.
In this heat roller system, a rubber layer consisting of, e.g., silicone rubber is formed on one of the rotary members in order to obtain a predetermined nip width.
It is considered preferable to use rubber having a high impact resilience as the rubber for this rotary member of a fixing device because such rubber is excellent in conveyance properties. For this reason, this sort of rubber is used in many black-and-white image forming apparatuses.
In full-color image forming apparatuses, on the other hand, it is preferable to use rubber layers on both fixing rollers of the roller pair because, in these apparatuses, multiple toner layers are formed and so the total thickness of toner layers is large.
In formation of full-color images on both surfaces of a recording medium, however, the use of high-impact-resilience rubber encounters the following problems.
An unfixed toner image of full color is formed by multiple layers of several color toners, and so the layer thickness of a fixed image is larger than that of a black-and-white image. Therefore, when a recording medium enters the nip portion in order to perform fixing of the second surface, the unevenness of the image already fixed on the first surface causes slight changes in angle and rate of entering and vibrations of the recording medium, thus disturbing the image on the second surface. In addition, since a full-color image consists of multiple layers as described above, formation of full-color images requires a larger heating amount than that for black-and-white images. Furthermore, in order to improve color mixing properties, a toner having good melt properties is used as a color toner. This increases the coating amount of a release agent for preventing offset with respect to the rotary members compared to those in apparatuses for black-and-white images. In the fixing device of a full-color image forming apparatus, therefore, there is a possibility that the surfaces of the rotary members deform under the influences of, e.g., long-term heating and a release agent, and the disturbance in an image described above becomes significant due to the deformation. That is, the fixing device of a full-color image forming apparatus is unsuitable for a long-term operation although the qualities of images are good in early stages.
It is an object of the present invention to provide a color-mixing fixing device capable of forming full-color images on both surfaces of a recording medium.
It is another object of the present invention to provide a color-mixing fixing device which does not disturb an image in color mixing on the second one of the two surfaces.
It is still another object of the present invention to provide a color-mixing fixing device in which the impact resilience of a rubber layer on the surface of each of a pair of fixing rotary members is 50% or less.
Other objects of the present invention will become apparent in the following description.